Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

INJECTABLE ANESTHETIC AGENTS
in heart rate frequently occurs. In healthy animals
the resulting hypotension is not severe but in patients
with congestive heart failure or hypovolemia, hypotension
can be profound.
● Administration of thiopental can be associated with
ventricular dysrhythmias and bigeminy is occasionally
seen. These arrhythmias do not appear to progress
to anything more serious and cease shortly after
the induction dose is given. During halothane anesthesia
thiopental reduces the threshold for adrenaline
(epinephrine)-induced cardiac arrhythmias. However,
thiopental does not increase sympathetic tone and is
suitable for induction of hyperthyroid cats provided
that heart failure is not present.
● The intravenous injection of greater than 2.5% thiopental
in small animals may result in thrombophlebitis
and intra-arterial administration will cause
arteriospasm and ischemic tissue necrosis. Perivascular
injection can cause tissue necrosis and sloughing.
When extravasation is suspected 0.5–2 mL of lidocaine
(lignocaine) without adrenaline (epinephrine)
should be infused around the area. Lidocaine (lignocaine)
is an acidic solution and this helps neutralize
the alkalinity of the barbiturate. More importantly,
it induces a localized vasodilation that promotes
uptake of the drug. Infiltration of the area with saline
to further dilute the barbiturate will also help.
Respiratory effects
● Thiopental produces dose-dependent respiratory
depression with a reduction in both rate and tidal
volume. Transient postinduction apnea is frequently
seen. Thiopental depresses sensitivity of the respiratory
center to carbon dioxide and the apnea is probably
due to high plasma concentrations that are present
after a bolus administration. Ventilation should be
assisted until spontaneous breathing resumes.
● During light thiopental anesthesia the laryngeal
reflexes remain strong and laryngospasm at intubation
may be seen in cats. Arytenoid function is minimally
depressed during light thiopental anesthesia,
making this agent useful for assessment of laryngeal
paralysis.
Hepatic effects
● High doses of thiopental can induce hepatic dysfunction,
but low doses appear to be safe even in patients
with liver damage.
● Thiopental is metabolized in the liver and a prolonged
recovery can be anticipated in the patient
with liver disease.
Renal effects
● When hypotension develops secondary to deep
or prolonged thiopental anesthesia a reduction in
renal blood flow occurs that can impair renal
function.
● Uremia may enhance the effect of thiopental and the
drug should be avoided or used with care in patients
with raised blood urea concentrations.
Other effects
● Thiopental easily diffuses across the placenta and
depresses the neonate during cesarean section.
Alternative induction agents with a shorter duration
of action should be considered for this surgery.
● Thiopental decreases intraocular pressure and can be
administered to patients with glaucoma, penetrating
eye injury, deep corneal ulcer or descemetocele.
● Engorgement of the reticuloendothelial system with
blood after thiopental administration makes it
unsuitable for splenectomy, liver biopsy and
tonsillectomy.
● Thiopental appears to reduce packed cell volume
(PCV) to a greater extent than other injectable
agents.
● Anaphylactic reactions to thiopental have been
reported, although such reactions appear to be rare.
Contraindications and precautions
Thiopental should be avoided or used with caution in
the following patients.
● Hypovolemic and septic patients
● Patients with cardiac disease, especially
dysrhythmias
● Uremic patients
● Patients with liver disease
● Patients undergoing splenectomy and liver biopsy
● Sighthounds
● Emaciated patients
Known drug interactions
● The dosage of thiopental required to induce anesthesia
is variably reduced by the prior or concurrent
administration of sedatives and opioids. If α 2 -
agonists are used for premedication the induction
dose may be reduced by 50% or more.
● Phenobarbital treatment stimulates hepatic microsomal
enzyme activity and hastens the metabolism of
thiopental in greyhounds; this effect may occur in
other breeds also.
Propofol
Clinical applications
Propofol is a very short-acting alkylphenol that is typically
used to induce anesthesia prior to maintenance
with an inhalational agent. Induction of anesthesia
is rapid and generally smooth. The transition to
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